Reading disability (RD) and attention-deficit/hyperactivity disorder (ADHD) are two ofthe most prevalent behavioral disorders of childhood and, thus, are important public health problems. Moreover, compared to children with either RD or ADHD alone, those who meet criteria for both disorders often have greater social and academic difficulties, and are at higher risk for other behavioral disorders. In order to contribute to a better understanding of the nature of RD, ADHD and their comorbidity, the primary objectives of proposed Project VI ofthe Colorado Learning Disabilities Research Center (CLDRC) are to 1) assess the developmental etiologies of RD, ADHD and their comorbidity; 2) assess the developmental relations among deficits in word reading, comprehension and written expression; and 3) assess key developmental outcomes in adolescence, including academic achievement, social functioning and comorbid psychopathology. To accomplish these objectives, we propose to conduct a five-year follow-up of twin pairs previously tested in the CLDRC, who were between 8 and 13 years of age at their initial assessment. An extensive psychometric test battery, including tests of reading performance, comprehension, written expression and various reading- related processes, cognitive abilities, and behavioral interviews and questionnaires will be administered to 116 twin pairs in which at least one member had a history of reading difficulties and/or ADHD at their initial assessment, to their siblings, and to 40 twin pairs in which neither member had a history of RD or ADHD. These data will be combined with follow-up data previously collected under separate funding (DC05190), as well as with data from the International Longitudinal Twin Studies of Early Reading Development (ILTS), recently funded to extend testing through ninth grade. Combining data from selected ILTS twin pairs and the CLDRC follow-up samples will facilitate more powerful tests ofthe developmental etiologies of reading/writing difficulties, ADHD and their comorbidity. This project will be the first to assess these longitudinal relations and outcomes within a genetically informative sample selected specifically for RD and/or ADHD symptoms. As such, it complements the overall CLDRC program theme in its application of behavioral genetic methods to the assessment ofthe developmental etiologies of reading/writing deficits, ADHD and their comorbidity and covariation with other reading-related cognitive measures.